I have a 1991 Dodge B250 Roadtrek with the 318 engine 727 transmission. It starts first time, idles properly, goes through the gears to high and runs smoothly until it reaches an incline. It will then misfire and the more I accelerate the worse it becomes, if I drop to a lower gear it will pull without a misfire. I have checked the distributor cap, completely dry, spark plugs are fine, no codes are shown on the light test. Any advice would be most welcome

 

Clogged cat?

 

Classic symptom of weak fuel pressure.

 

fuel pump tested, good pressure, over 9, manifold vacuum checked, all ok,